1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to circuit breakers. The disclosed concept also pertains to latch assemblies for such circuit breakers.
2. Background Information
Latches are an important part of electrical switching apparatus, such as circuit breakers, and can take the form shown in FIG. 1. A latch assembly 2 includes three components: a D-shaft 4, a latch plate 6 and a latch shaft 8. The latch plate 6 and the latch shaft 8 are joined together (i.e., by copper brazing; by soldering; by welding followed by heat treating) and the latch plate 6 rotates about the longitudinal axis 9 of the latch shaft 8. The D-shaft 4 blocks (near D-shaft slot 10 when in a corresponding range of suitable axial positions), as shown, or allows the movement of the latch plate 6 (through the D-shaft slot 10 when in a corresponding suitable axial position (not shown)) as the D-shaft 4 rotates (e.g., clockwise with respect to FIG. 1) on its longitudinal axis 9. The latch shaft 8 and the D-shaft 4 both rotate about their respective longitudinal axes, which are disposed a fixed distance apart. The latch assembly 2 can only rotate clockwise (with respect to FIG. 1) when the D-shaft 4 is suitably oriented (not shown) to allow the latch plate 6 to pass through the D-shaft slot 10.
In order to optimize performance, the strength and the hardness of the latch plate 6 and the latch shaft 8 are carefully controlled, preferably inexpensively. For example, the latch plate 6 is made of 420 stainless steel and the latch shaft 8 is made of 410 stainless steel. These are copper brazed together and heat treated at the same time. However, the stainless steel will not get hard enough to hold up to life testing and, thus, the tip 12 of the latch plate 6 can deform. Also, a problem with copper brazing certain types of steel, such as 1070 stainless steel, is that the copper braze would melt when such steel is heat treated and, hence, the parts would not stay together in a heat treatment furnace.
Known current practices of rigidly joining the latch plate 6 to the latch shaft 8 include brazing or welding operations. However, for example, cracks in the welds after heat treatment can be an issue with welded parts. These operations present severe limitations on the choice of materials for these components and corresponding heat treatment options. The processes available with acceptable materials limit the component hardness and/or are difficult to control in a repeatable manner in an industrial environment. Hence, there is room for improvement in latch assemblies.
There is also room for improvement in electrical switching apparatus, such as circuit breakers, including a latch assembly.